Zinc is the second most abundant transition metal ion in the human body, where it has multiple roles in both intra and extra cellular functions. A large number of proteins and enzymes were identified to contain Zn2+. Zinc is reported to be responsible for neurological disorders such as Alzheimer's disease (AD), amyotropic lateral sclerosis (ALS), Parkinson's disease and Epilepsy. Alzheimer's disease is a neurodegenerative disease associated with aggregation of β-amyloid peptides (Aβ) in the brain. Aβ1-42 has a high affinity to metal ions and forms insoluble aggregates particularly in the presence of metal ions like Zn2+, Cu2+ and Fe2+. In addition, zinc plays crucial role in insulin secretion and apoptosis. The World Health Organization estimates that more than 40% of the children in Africa and Asia have stunned growth associated with limited dietary zinc. The extent to which zinc deficiency conditions persist today is difficult to estimate because of the lack of suitable biochemical markers for zinc.
Besides growth, numerous body functions are affected by zinc ions, including immune, endocrine and gastro-enterological systems. The huge scope for the exploration of the diverse physiological roles of biological zinc demands sensitive and non-invasive technique for the real time detection and imaging. The relative concentration of free Zn2+ within biological cells varies from 1 nM in the cytoplasm of many cells to 1 mM in the vesicles of presynaptic neurons in human brain. Although the total concentration of the zinc in a cell is relatively high, the concentration of the free zinc, which is not strongly bound to proteins, is extremely low. The estimation of the free zinc has been proved to be difficult using classical methods. These concerns have been on the top priority of chemists to develop selective and efficient probes or the so called chemosensors for zinc ions.
Since Zn2+ is silent to most of the analytical techniques, fluorescent techniques stand out as a method of choice. This method utilizes a probe molecule that recognizes Zn2+ and emits specific wavelength upon binding, which in turn allows tracking of zinc ions in live cells using fluorescence microscopy. A fluorescent molecular probe consists of a fluorophore attached to a chelating agent or an ionophore with or without a spacer group. A change in the fluorescence intensity or wavelength occurs as a result of an analyte binding which results in a signal output and can be studied spectroscopically. An effective chemosensor must convert the event of cation recognition by the ionophore into an easily monitored and highly sensitive light signal from the fluorophore. References may be made to: a) P. Jiang, Z. Guo, Coord. Chem. Rev. 2004, 248, 205-229; b) N. C. Lim, H. C. Freake, C. Brückner, Chem.-Eur. J. 2005, 11, 38-49; c) J. M. Berg, Y. Shi, Science 1996, 271, 1081-1085; d) M. P. Cuajungco, G. J. Lees, Neurobiol. Dis. 1997, 4, 137-169; e) M. Mattson, Nature, 2004, 430, 631; f) M. Citron, Nat. Rev. Neurosci, 2004, 5, 677; g) M. de Onis, E. A. Frongillo, M. Blössner, Bull. World Health Organ. 2000, 78, 1222-1233; h) P. Carol, S. Sreejith, A. Ajayaghosh, Chem. Asian J., 2007, 2, 338-348.
Reference may be made to Ajayaghosh, P. Carol, S. Sreejith, J. Am. Chem. Soc., 2005, 127, 14962-14963 wherein the fluorophores 2a-c (formula 2) showed strong emission around 537 nm in acetonitrile with a quantum yield of 0.4. In buffered (HEPES, pH=7.2) acetonitrile-water mixture (9:1 v/v), titration of transition metal salts to 2c showed strong quenching of the emission at 547 nm except in the case of Zn2+ which resulted in a red-shifted emission at 637 nm. Alkali and alkaline earth metal salts could not induce any considerable changes to the emission behavior of 2a-c. The binding of Zn2+ was highly selective in the presence of a variety of other metal ions. Though Cu2+ quenches the emission of 2c, in the presence of Zn2+ a red emission prevails indicating the preference of 2c towards Zn2+. The selective visual sensing of Zn2+ with a red emission is ideally suited for the imaging of biological specimens.

However, the other derivatives of formula 2a-c, act as ratiometric sensors for Zn2+ ions only in organic/aqueous solvents and not in the solid state and this prevented the practical use of these derivatives.
The main disadvantages which have prevented the practical application of molecular probes for the detection of metals ions in aqueous media at normal conditions are;
1) The first disadvantage is that the detection still requires professional laboratory type operations, such as precise transfer of solutions, usage of sophisticated instrumental techniques etc., making it less useful to people who do not have any scientific background.
2) Second is the low sensitivity and selectivity Of the probes towards metal ions for an instrument free observation of binding events. For example, distinguishing analyte binding event by color change or fluorescence change with naked eye.
3) Third one is related to the property of the fluorophore. The selected fluorophore must be water insoluble and at the same time it should interact with metal ions in the solution. An ideal fluorophore must have comparatively good quantum yield in solid state and
4) The fourth and significant one is the difficulty in reprocessing and thereby multiple use of the probe.